1. Field of the Invention
The present invention relates to a lead screw used in a linear actuator, and a method of manufacturing the same.
2. Description of the Prior Art
A linear actuator having a lead screw is set in, e.g., a floppy (registered tradename) disk drive unit (FDD), and is used to linearly drive a carriage with a magnetic head so as to access a desired track.
FIG. 1 is a perspective view showing the schematic arrangement of a floppy (registered tradename) disk drive unit 50 with a linear actuator 40 to which a conventional lead screw 30 is attached.
As shown in FIG. 1, in the floppy (registered tradename) disk drive unit 50, a pressure bar spring 42 extending from a carriage 51 supports the lead screw 30 and holds a needle 43 pressing against a groove formed in the lead screw 30. Thus, when the lead screw 30 is rotated by a stepping motor 41, the carriage 51 with a magnetic head 52 slides on a guide bar 53 to move linearly. The magnetic head 52 mounted on the carriage 51 can access a track on a desired magnetic disk (not shown) upon forward/reverse rotation of the stepping motor 41.
FIG. 2 is a side view of the linear actuator 40 in which the lead screw 30 is attached to the stepping motor 41.
As shown in FIG. 2, the stepping motor 41 is attached to a side plate 55 of the floppy (registered tradename) disk drive unit 50 (see FIG. 1). One end of the lead screw 30 is supported by a pivot bearing (not shown) in the stepping motor 41, and the other end thereof is supported by a pivot bearing 45 attached to a base 44 to which the stepping motor 41 is fixed.
Conventionally, the lead screw 30 for use in the floppy (registered tradename) disk drive unit 50 has been formed by machining a predetermined helical thread as required in a metal rod made of brass or stainless steel.
Although the conventional lead screw 30 made as described above has a high dimensional precision, its machining method is complicated, and its manufacturing cost is high. Furthermore, it does not have a good productivity. As a result, it can not be provided at a low cost.
In order to solve the above problems, a lead screw having a resin-molded thread has been put into practical use, as is proposed by the present applicant (assignee) and disclosed in Japanese Unexamined Patent Publication No. 8-118371.
FIG. 3 is a sectional view showing the main part of a manufacturing apparatus for a conventional lead screw 60 having a resin-molded thread 63.
A metal pipe 61, which has in its wall portion 61a penetrated holes 62 arrayed helically at predetermined intervals so as to correspond to the thread 63 to be resin-molded, and a metal mold 69, which has on its inner surface a helical groove 68 (for shaping the thread 63) corresponding to the array of the penetrated holes 62, are placed in the manufacturing apparatus. The metal pipe 61 is set in a cavity 67 of the mold 69 such that the array of the penetrated holes 62 and the helical groove 68 correspond to each other. When the lead screw 60 is manufactured, a resin material is injected into the metal pipe 61 so that the resin material flows into the cavity 67 through the penetrated holes 62, so that the thread 63 is resin-molded. With this technique, a high-precision, inexpensive lead screw can be manufactured.
FIG. 4 is a side view showing the conventional lead screw 60 having the resin-molded thread 63.
As shown in FIG. 4, the lead screw 60 is supported, through a ball 66, by a pivot bearing 45 fixed to a base 44.
The interior of the metal pipe 61 is simultaneously filled with a resin 64 when the thread 63 is resin-molded. The ball 66 is in contact with the resin 64.
The lead screw 60 has no problem in its pitch characteristics, but the position of its origin slightly changes depending on the ambient temperature (normal or high (e.g., 80° C.)) due to the temperature characteristics, that is, the coefficient of thermal expansion of the resin 64. For this reason, the distance of linear movement of the lead screw 60 upon rotation of the stepping motor may vary slightly.